Water treatment systems have been in existence for many years. These systems treat stormwater surface run-off or other polluted water. Stormwater surface runoff is of concern for two main reasons: one because of the effects of its volume and flow rate, and two, because of the pollution and contamination it can carry. The volume and flow rate of stormwater is important because high volumes and high flow rates can cause erosion and flooding. Pollution and contamination are important because stormwater is carried into our rivers and streams, from there into our lakes and wetlands, and furthermore because it can eventually reach our oceans. Pollution and contamination that is carried by stormwater can have adverse affects on the health and ecological balance of the environment.
Beginning in 1972 with the passage of the Clean Water Act the federal government through the Environmental Protection Agency has mandated progressively tighter controls over the quantities of pollutants and contaminants that are allowed to be released into our nation's waters. These progressively tighter mandates also encompass control of peak flows and/or volumes and the rate at which they can be discharged into existing water ways or drainage infrastructures. These resulting mandates and controls have given birth to new programs and procedures collectively referred to as stormwater management. Devices and procedure that remove or reduce the pollutants and contaminates and/or control peak flows and volumes are often referred to as best management practices or BMPs. BMPs utilize natural means, artificial or man-made means, and even combinations of either and/or both. Some examples of these BMPs include trash filters, sedimentation basins, retention and detention ponds, wetlands, infiltration trenches, grass swales, various types of media filters, and various types of natural filter systems including sand filters, and aggregate filters including natural and artificial wetlands. These BMPs typically use one or more mechanisms to remove the pollutants and contaminates. These mechanisms include sedimentation, filtration, absorption, adsorption, flocculation, stripping, leaching, bioremediation, and chemical process including oxidation reduction, ion exchange, and precipitation.
Furthermore, stormwater treatment systems can also be classified in relationship to the treatment level in which they are being used. In this respect the term treatment is generally used to describe the unit processes that are used to reduce the quantities of pollutants and containments in stormwater runoff. For example, basic or pre-treatment typically refers to the removal of gross solids, sediments and larger debris through the processes of settling and screening, while enhanced or advanced treatment typically refers to processes for reducing targeted pollutants; filtration being the main form of enhanced treatment for stormwater. Filtration utilizes a combination of physical, chemical, and biological processes. Types of filtration greatly vary dependent on the media use. Medias can be both inert and/or sorbent and are also strongly linked to natural biological processes that thrive in and/or around the media environment. Advanced filtration techniques especially include chemical and biological processes and generally include, but are not limited to processes that bring stormwater in contact with plants including both macrophytes and microphytes, plants that are both visible and invisible to the naked eye. One type of stormwater treatment system that is especially effective at advanced treatment is known as a wetlands system or often simply referred to as a wetlands.
When creating a constructed wetlands, the objective is to minimize the size of the media to maximize the surface area of the media and to also maximize contact time with possible biofilm which can grow on the media, but also to provide media sufficiently large so that the interspacing will not be occluded with the sedimentation that is being carried in the treated water. Accordingly, as a matter of practicality it makes sense to remove as much sediment as possible before allowing the water to enter the wetlands system. In this respect the design of an effective treatment system would contain sufficient screening to remove trash and debris, sufficient sedimentation to remove sediment to a level sufficient to maximize the use of the wetlands. And to preserve efficient operation of the system, the system should be operated at an appropriate flow rate that maintains and preserves the life and operation of the system as a whole. The average or mean time that water remains in contact with the wetlands system is termed the hydraulic resident time or HRT of the wetlands.
Given uniform flow through the sediment chamber, the sedimentation HRT is proportional to the volume of the chamber and inversely proportional to the flow rate. The time required for a particle to settle a specific distance is often referred as the settling time for that particle size and density. Because deeper settling chambers require a greater distance for particles to settle, deeper settling chambers have longer respective settling times. And, because the volume of a sediment chamber is also proportional to the depth of the chamber, increasing the depth increases both the volume (and thereby the HRT) and the settling time. Therefore, increasing the depth of the chamber increases the HRT, but may not increase settling efficiencies since the distance to settle increases proportionally with increase in HRT. Accordingly, the overriding principle of achieving effective sedimentation is to provide the maximum surface and floor areas in the chamber as possible. Other considerations are to increase the path length through the sediment chamber to increase the uniformity of the flow and to prevent high flow rate conditions from re-suspending existing sediment (often referred to as scouring).
In a similar manner, the basic principles separation that apply to the settling of particles more dense than water apply to particles that are less dense than water except that the particles float to the surface rather than settle to the bottom of the chamber. Because oils and hydrocarbons are typically less dense than water, because these products can often be separated mechanically by flotation, and because the products can create adverse biological demands on a wetlands system placed downstream of the sedimentation and flotation chamber, it makes sense to allow the floatable products to remain in the chamber and to remove the out-flow water from below the surface.
Because the objective of a sedimentation and flotation chamber is to remove sediment and floatable products from the incoming water, the accumulated sediment and floatable products will require periodic removal. Systems that are configured to allow easy removal of these products will undoubtedly provide reduced maintenance costs.
Flow-rate control is another consideration. Because the performance of some BMPs like sedimentation and flotation chambers and wetlands systems is dependent on hydraulic resident times (HRTs), optimum performance can be obtained by having sufficient control to not allow flow rates to vary excessively beyond certain limits. Devices that can be used to control the flow rate include bypass controls and inlet and outlet control systems such as adjustable valves or orfice plates.
Because some treatment locations may have high levels of specific pollutants and contaminates, specific configurations using additional BMPs may provide benefits and advantages above typical or standard configurations. Such BMPs may include additional filter systems, additional media chambers, aeration, bioremediation systems, irrigation systems, and mixing manifolds. Any system that can be configured to accommodate additional devices and methods of pollution and contaminate removal is by definition a more versatile BMP.
There is, thus, a need for a wetlands stormwater treatment system which can treat high levels of specific pollutants and contaminants.